The JS strain of PIV3 is the wild type parent of three candidate live attenuated cold-passaged (cp) viruses; namely cp12, cp18, and cp45. In order to develop an understanding of the genetic basis of attenuation of the mutant viruses, the nucleotide and deduced amino acid sequence of the JS wt and cp12 and cp45 mutant viruses was undertaken. The complete sequence of the JS wild type PIV3 has been obtained and that of the cp12 and cp45 mutants is almost completed. The most promising attenuated vaccine candidate, the cp45 virus, contains at least 16 nucleotide changes from the JS wild type virus (a few nucleotide differences remain to be confirmed). We are using the cDNA clones employed to sequence the JS wild type and cp45 viruses to construct complete cDNA copies of each of these two viruses, a project that is 70% complete. A strategy is being developed to produce an infectious virus from RNA transcribed from the complete copy of PIV3 which will greatly accelerate our ability to produce live attenuated PIV3 vaccines.